Use a mutex guard when the unprotection is triggered from a compaction
space in which case it is actually parallel.

Main-thread only unprotection does not require acquiring the mutex.

The list itself is only used from the main thread and thus the actual
process does not require a mutex.

The issue was introduced in https://crrev.com/c/2966382

Bug: v8:11982
Change-Id: I593c0659eb5a96c8206d0b4014f07ab13827be85
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3026705Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75734}